1. Field of the Invention
The present invention relates to a high-frequency switch, and particularly, relates to a high-frequency switch for switching a signal path in a high-frequency circuit, for example, a digital portable telephone or the like.
2. Description of the Prior Art
As shown in FIG. 8, a high-frequency switch is used for switching a connection between a transmitting circuit TX and an antenna ANT and a connection between a receiving circuit RX and the antenna ANT in a digital portable telephone or the like.
FIG. 9 is a circuit diagram showing one example of a high-frequency switch which is a background of the present invention. The high-frequency switch 1 is connected to an antenna ANT, a transmitting circuit TX and a receiving circuit RX. An anode of a first diode 3 is connected to the transmitting circuit TX via a capacitor 2. The anode of the first diode 3 is grounded via a series circuit of a first strip line 4 and a capacitor 5. A first control terminal 6 is connected to a connecting point of the first strip line 4 and the capacitor 5. A control circuit for switching the high-frequency switch is connected to the first control terminal 6. Also, a cathode of the first diode 3 is connected to the antenna ANT via a capacitor 7. A series circuit of a strip line 8 as an inductor and a capacitor 9, and a resistor 10 are connected to the first diode 3 in parallel. Furthermore, one end of a second strip line 11 is connected to the antenna ANT via the capacitor 7. The other end of the second strip line 11 is connected to the receiving circuit RX via a capacitor 12. An anode of a second diode 13 is connected to a connecting point of the second strip line 11 and the capacitor 12. A cathode of the second diode 13 is grounded via a capacitor 14. A resistor 15 is connected to the second diode 13 in parallel. Furthermore, a second control terminal 17 is connected to the cathode of the second diode 13 via another resistor 16. Another control circuit for switching the high-frequency switch is connected to the second control terminal 17.
When transmitting with the high-frequency switch 1 shown in FIG. 9, a positive voltage is applied to the first control terminal 6 so that the potential of the first control terminal 6 becomes higher than that of the second control terminal 17. At this time, a D.C. component is blocked by the capacitors 2, 5, 7, 12 and 14, so that the voltage applied to the first control terminal 6 is applied to only a circuit including the first diode 3 and the second diode 13. Thus, by the voltage, the first diode 3 and the second diode 13 are turned ON. When the first diode 3 and the second diode 13 are ON, a signal from the transmitting circuit TX is transmitted to the antenna ANT, and then the signal is transmitted from the antenna ANT. Also, at this time, as showing an equivalent circuit when the first diode 3 and the second diode 13 are ON in FIG. 10, inductance components exist in the first diode 3 and the second diode 13. Thus, the transmitting signal from the transmitting circuit TX is not transmitted to the receiving circuit RX, since the second strip line 11 is grounded via the second diode 13 and the capacitor 14 and resonates, and an impedance observed from the connecting point A to the receiving circuit RX side is infinite.
On the other hand, when receiving, the voltage is not applied to the first control terminal 6 and a positive voltage is applied to the second control terminal 17, so that the first diode 3 and the second diode 13 are turned OFF. Thus, a receiving signal is transmitted to the receiving circuit RX and is not transmitted to the transmitting circuit TX side.
In this way, in the high-frequency switch 1, by controlling voltages applied to the first control terminal 6 and the second control terminal 17, transmitting and receiving can be switched.
However, in the conventional high-frequency switch 1 shown in FIG. 9, when receiving, as showing an equivalent circuit when the first diode 3 and the second diode 13 are OFF in FIG. 11, capacitance components exist in the first diode 3 and the second diode 13. When such a capacitance component exists in the second diode 13, since the other end of the second strip line 11 is grounded via the capacitance component, the characteristic impedance of the second strip line 11 becomes low and does not match with the impedance of the receiving circuit RX side. Thus, in the high-frequency switch 1, as showing a reflection characteristic between the antenna and the receiving circuit at receiving in FIG. 12, there is a problem that a reflection loss at receiving is large and an insertion loss at receiving is large as 1.0 dB.
Meanwhile, in the high-frequency switch 1, when the second diode 13, the capacitor 14, the resistors 15 and 16 are removed for decreasing the insertion loss at receiving, as showing a reflection characteristic between the antenna and the receiving circuit at receiving in FIG. 13, though the reflection loss and the insertion loss become small at receiving, it is not used for a high-frequency switch since a signal flows to the receiving circuit RX side at transmitting.